Method for steam drying filter cake

ABSTRACT

A method is disclosed for steam drying filter cake on a drum or disc type vacuum filter having a steam drying cycle wherein optimum cake thickness and moisture content is obtained at optimum thermal efficiency by controlling steam condensation within the cake at constant steam input, through control of the vacuum on the filter during cake formation and during filter cake drying with generally constant speed of rotation of the filter element.

United States Patent Emmett, Jr. et al.

METHOD FOR STEAM DRYING FILTER CAKE Inventors: Robert C. Emmett, Jr., East Dundee, lll. 60l l8; Donald A. Dahlstrom, Salt Lake City, Utah 84117 Assignee: Envirotech Corporation, Salt Lake City,

Utah

Filed: 0cl. 30, 1970 Appl. No.1 85,488

U.S. Cl ..34/15, 34/51, 34/92 [56] References Cited 7 UNITED STATES PATENTS 3,186,103 6/1965 Burton ..34/l5 Primary ExaminerFrederick L. Matteson Assistant Examiner-W. C. Anderson Attorney-Stowe & Stowell [57] ABSTRACT A method is disclosed for steam drying filter cake on a drum or disc type vacuum filter having a steam drying cycle wherein optimum cake thickness and moisture content is obtained at optimum thermal efficiency by controlling steam condensa- Int. Cl ..F26b 5/04 tion within the cake at constant steam input, through control Field of Search ..34/ 15, 16.5, 50, 51, 92, 37; of the vacuum on the filter during cake formation and during 210/63 97 filter cake drying with generally constant speed of rotation of the filter element.

5 Claims, 3 Drawing Figures PATENTEDJUN 27 I972 sum 10F 2 FIG.I

INVENTOR ROBERT C. EMMETT JR.

DONALD A. DAHLST OM J'QM/A ATTORNEY? METHOD FOR STEAM DRYING FILTER CAKE Related subject matter is disclosed and claimed in U.S. patent application Ser. No. 86,180 Method for Steam Drying Case I Emmett and Dahlstrom filed even date herewith;

application Ser. No. 85,321 entitled Apparatus for Steam Drying Filter Cake R. C. Emmett, Jr. et al. filed even date herewith and U.S. patent application Ser. No. 692 filed Jan. 5, 1970 entitled Method and Apparatus for Steam Drying Filter Cake now U.S. Pat. No. 3,592,341.

BACKGROUND OF INVENTION The invention relates generally to drying of filter cake on rotary drum or disc type vacuum filters provided with a steam dome or hood whereby during at least a portion of the drying cycle steam is passed through the formed filter cake and condensation of the steam within the cake releases heat to lower viscosity of the water contained in the filter cake.

The use of steam as an aid for reducing moisture content of filter cake is generally well known and tests have established that the viscosity of water falls from for example 1.3l to 0.28 centipoises when the temperature of the water increases from to about 100" centigrade. Such reduction in viscosity brings about better draining efficiency and a considerable lowering of the residual water content of the filter cake.

It has also been recognized in the art that to optimize thermal efficiency of steam dewatering processes on rotary drum and disc filters, the proper location of the condensation front of the steam is within the filter cake and preferably continuing until it is adjacent the surface of the filter media. Notwithstanding such teachings of the prior art, apparatus for steam drying filter cake on rotary drum and rotary disc type vacuum filters have not operated at optimum efficiencies as the systems lack the necessary control to maintain high filtration rates and high thermal efficiencies under varying conditions such as particulate concentration of the slurry feed, the size and nature of the particulate material, size distribution of the treated product and cake thickness.

OBJECTS OF THE INVENTION It is a primary object of this invention to provide a method or system for controlling steam drying rotary vacuum filters whereby optimum cake thickness and moisture content and optimum thermal efficiency is readily achieved and maintained.

A further object of the invention is to provide such a method or system that is commercially feasible when applied to continuous systems handling slurries of variable content.

A further object of the present invention is to provide such a system wherein the control means include a plurality of parameters thereby substantially extending the range of operation of apparatus constructed to carry out the method of the invention.

These and other objects and advantages of the invention are provided in a method for drying a filter cake formed by continuous filtration of a feed slurry on a closed hood type rotary vacuum filter comprising:

1. Passing steam through the filter cake in a zone above the surface of the feed slurry;

2. Controlling condensation of the steam within the filter cake by applying a vacuum to the under surface of the filter cake in said zone;

3. Continuously monitoring and controlling the applied vacuum during cake formation and filter cake drying while maintaining the speed of rotation of the filter element substantially constant to maintain optimum thennal efficiency of the applied steam.

The invention will be more particularly described in reference to the accompanying drawing wherein:

FIG. 1 is a schematic diagram of a system adapted for carrying out the present invention;

FIG. 2 is a fragmentary end view of the filter apparatus illustrated in FIG. 1 showing the control and drive means for rotating the filter unit; and

FIG. 3 is a chart illustrating the passage of saturated or superheated steam through a filter cake at insufficient steam application, proper steam application and excessive steam application.

Referring to the drawing and in particular to FIGS. 1 and 2 thereof 10 generally designates a rotary disc type vacuum filter including a central hub 12 which supports a plurality of conventional filter discs 14 of the type shown in our copending application Ser. No. 692 filed Jan. 5, 1970. The filter 10 includes a tank or trough portion 16 and a steam dome or hood 18. Each of the discs of the disc filter is connected via a conventional filter valve 20 to a source of vacuum via a filter cake form receiver generally designated 22 and a drying cycle vacuum receiver generally designated 24, to be more fully described hereinafter. The steam dome or hood 18 is supplied with superheated steam from a source generally designed 26 via automatic control valve 28 and valved conduits 30, 32 and 34 which discharge the steam in a controlled pattern adjacent the inner surface of the top of the dome as more fully described in our application Ser. No. 85,321 filed even date herewith.

The steam dome or hood 18 is also provided with a visual thermometer 36 registering the temperature within the dome.

Remote temperature sensing is provided via sensor 37 and line 38 which is connected to the control panel generally designated 40. Further an over pressure 'or temperature alarm line 42 having sensor 43 is connected to an alarm 47 on the panel and a further switch 46 adjacent the filter 10.

The filter cake form receiver 22 is connected to a vacuum pump or other source of reduced pressure via conduit 50. The conduit 50 is joined to the form receiver 22 via a large control valve 52 provided with an electrically or pneumatically controlled actuator 54 which actuator is connected to the control station 40 via line 56. The form receiver 22 is also provided with a control valve 63 of the pneumatically or electrically actuated type and the power for actuation of the valve 63 is derived from the control station 40 via line 65, whereby valve settings for cake formation may be determined and controlled at the remote station 40. Primarily, the large valve 52 is employed only during start-up while control of the cake formation during operation is via small valve 63 with the large valve 52 closed or substantially closed.

The pressure within the form receiver 22 is constantly sensed and the sense pressure is relayed to the control station via conduit 58. As hereinbefore set forth, the form receiver 22 is connected to the filter valve 20 via filtrate conduit 60 and valve coupling 62, whereby essentially only the portions of the filter discs 14 below the level of the slurry in tank 16 are connected to vacuum via the form receiver 22.

The drying cycle vacuum receiver 24 is connected to a source of reduced pressure via conduit 64. The conduit 64 is provided with a control valve 66 of the electrically or pneumatically actuated type and the power for actuation of the valve 66 is derived from the control station 40 via line 68. The receiver is also provided with a pair of temperature sensors 70 and 72. Sensor 70 is connected via switch 74 to the control panel 40 and the switch 46. Sensor 72 is connected to the control panel or station 40 via line 76 which in addition to being connected to the dry receiver temperature recorder 78 is connected to control line 68. Thus opening of valve 66 which in turn determines, within limits, the vacuum drawn on the filter during the cake drying cycle assists in maintaining condensation front of the steam within the filter cake to optimize thermal efficiency of the system.

The filter valve 20 is connected to the dry receiver 24 via conduit 80 and valve coupling 82 whereby the filter sectors of the discs 14 are only connected to the receiver 24 during that portion of their rotary cycle when the cake is being dried and steam is passing through the fonned filter cake.

Referring to FIG. 2 of the drawing the shaft carrying the plural discs of the disc filter is provided with a large worm gear 92 which is driven by worm 94 connected through a gear box 96 to an electric motor 98. The electric motor is of the variable speed type and a controller for the electric motor 98 is provided and comprises final controller 100, a liquid level sensor 102 for controller 100. Controller 100 and sensor 102 are connected to the main control panel 40 via electrical conduit 104 via speed control indicator 106. The slurry for the tank is directed to feed trough 1 10 of conventional design. In general, in this form of the invention the filter rotates at constant speed varied only when the liquid level in the tank 1 10 falls below or exceeds a predetermined level.

Referring to FIG. 3, the chart illustrates at 116 the desired position of the steam condensation front through a filter cake under desired vacuum conditions such that steam in heating the filter cake is all condensed substantially at the line of intersection between the bottom of the cake and the filter media or in the zone of the filter medium. Line 118 shows the efiect of excessive steam application in which instance uncondensed steam 120 is drawn into the dry receiver 24 raising the temperature within the receiver and materially lowering the thermal efficiency of the system. With excessive steam application the rise in temperature is sensed by sensor 72 which sensed temperature rise is directed to the control panel 40 where a controller which may include an electrical bridge directs valve 66 in vacuum line 64 toward the closed position to decrease the vacuum on the filter cake dry receiver to a lower limit of about inches of mercury. If the sensed temperature is still too hot, controller 40 increases the vacuum on the form receiver via line 65 and valve 63. Reducing the vacuum on the dry receiver 24 reduces the rate at which steam is drawn from the dome through the cake. Further, by increasing the vacuum drawn on the form receiver 22 increases cake thickness which increases the distance of travel of the steam drawn by the dry receiver 24 through the filter cake. The dual controls thus provide a wide range of control to maximize thermal efficiency of the process.

Line 120 on the chart illustrates the effect of insufficient steam application and in this event, the temperature in the dry receiver 24 falls below a predetermined amount, controller 40 directs a signal to valve 66 in vacuum line 64 to increase the vacuum within receiver 24 and thereby increase the rate of steam passage through the filter cake. If the temperature remains too low with the valve fully opened, then controller 40 via line 65 and valve 63 reduces the vacuum on form receiver 22 to reduce the thickness of the filter cake. Controls for the valve 63 and 66 may be of the cyclic timed type such that power to the controllers only takes place for example 5 percent of each minute to reduce over control or hunting in the system. The specific form of controller, whether it will be of the anti-hunting type or not, forms no specific part of the present invention and it is contemplated that various types of conventional commercial controllers could be employed to bring about the desired results. While such controllers form no specific part of the present invention, the controllers illustrated for example in the following patents are considered to be satisfactory for carrying out the principles of the invention in conjunction with the apparatus herein described: U.S. Pat. No. 3,240,428 Umrath and U.S. Pat. No. 3,036,188 Ditto.

In general, tests have indicated that when filtering slurries containing fine materials cake thickness in the order of onefourth to one-half inch is about optimum while with coarse material cake thickness of one-half to as great as two inches can be satisfactorily handled. Fine materials such as clean fine coal as used herein are those wherein 30 to 40 percent or more of the particles are minus 200 mesh and above this range would be considered coarse material. With materials such as aluminum trihydrate or iron the percentage of small particles (-200 or below) may be as high as 95 percent. Further, it has been found that very useful operation is provided using dry steam at about 225F to about 300F when the steam is introduced to the steam hood at slightly above atmospheric pressure and at the amount of 50 to 100 Kg. of steam per metric ton of filter cake for minerals and coal having moisture contents of 20 to 25 percent bfivlsi%rt or less.

EXA L A slurry consisting of fine coal in water wherein about 40 percent of the fine coal is -200 mesh is fed to the tank 16 of a plural element rotary disc vacuum filter. Steam slightly superheated to about 250F. is directed to the hood at a pressure slightly above atmospheric eg. at about three pounds per square inch and at the amount of 50 to 100 Kg. of steam per metric ton coal fines. The supply is regulated via control valve 28 and the distribution within the hood is controlled by manual valves in line 30, 32 and 34. The start-up of the filter is without automatic control and the system is manually regulated such that the cake formed on the filter is about one inch thick at the end of the drying cycle. With the drum rotating at a speed commensurate with the vacuum on the vacuum form receiver 22 to provide this rate of cake formation, the system is switched to automatic control with the temperature sensor 72 set so as maintain a temperature within the dry receiver 24 of about 125F. With the automatic control in operation, the vacuum directed to the form receiver 22 via valve 66 and the vacuum directed to the dry receiver 24 is automatically controlled to maintain the temperature within the receiver 24 at the controlled temperature, thereby maximizing steam drying efiiciency for minimum final moisture content of the unit with the production of maximum filter cake per square foot of filter surface.

From the foregoing description of an embodiment of the present invention, it will be seen that the aims and objects herein set forth and others which will be apparent to those skilled in the art are fully accomplished.

We claim:

1. A method for drying a filter cake formed by continuous filtration of a feed slurry on a closed hood type rotary vacuum filter comprising:

a. passing steam at a constant input rate through the filter cake in a zone above the surface of the feed slurry;

b. controlling condensation of the steam within the filter cake by applying a vacuum to the under surface of the filter cake in said zone;

c. continuously monitoring and controlling the applied vacuum to the filter cake in said zone to maintain optimum thermal efficiency of the applied steam.

2. A method for drying a filter cake formed by continuous filtration of a feed slurry on a closed hood type rotary vacuum filter comprising:

a. continuously monitoring and controlling the vacuum applied to the filter element in a zone below the surface of the feed slurry to maintain an optimized cake thickness in a zone above the surface of the feed slurry;

b. further controlling the filtration rate by continuously monitoring and controlling the vacuum applied to the filter cake in a zone above the surface of the slurry;

c. directing live steam in the zone above the feed slurry at a rate such that substantially all of the steam is condensed in passing through the filter cake to maintain the condensation front in the zone of the filter medium.

3. The method defined in claim 2 wherein the steam passed to the filter cake is at a temperature of about 225 to 250 fahrenheit and the steam is passed at an amount of 50 to 100 Kg. of steam per metric ton of filter cake.

4. The invention defined in claim 3 wherein the dry filter cake is from about one-fourth to 2 inches in thickness.

5. The invention defined in claim 4 wherein the formed cake is about one-fourth to about one-half inch in thickness when the particles in the slurry to be filtered are in the range of 30 to 40 percent or more at -200 mesh. 

1. A method for drying a filter cake formed by continuous filtration of a feed slurry on a closed hood type rotary vacuum filter comprising: a. passing steam at a constant input rate through the filter cake in a zone above the surface of the feed slurry; b. controlling condensation of the steam within the filter cake by applying a vacuum to the under surface of the filter cake in said zone; c. continuously monitoring and controlling the applied vacuum to the filter cake in said zone to maintain optimum thermal efficiency of the apPlied steam.
 2. A method for drying a filter cake formed by continuous filtration of a feed slurry on a closed hood type rotary vacuum filter comprising: a. continuously monitoring and controlling the vacuum applied to the filter element in a zone below the surface of the feed slurry to maintain an optimized cake thickness in a zone above the surface of the feed slurry; b. further controlling the filtration rate by continuously monitoring and controlling the vacuum applied to the filter cake in a zone above the surface of the slurry; c. directing live steam in the zone above the feed slurry at a rate such that substantially all of the steam is condensed in passing through the filter cake to maintain the condensation front in the zone of the filter medium.
 3. The method defined in claim 2 wherein the steam passed to the filter cake is at a temperature of about 225* to 250* fahrenheit and the steam is passed at an amount of 50 to 100 Kg. of steam per metric ton of filter cake.
 4. The invention defined in claim 3 wherein the dry filter cake is from about one-fourth to 2 inches in thickness.
 5. The invention defined in claim 4 wherein the formed cake is about one-fourth to about one-half inch in thickness when the particles in the slurry to be filtered are in the range of 30 to 40 percent or more at -200 mesh. 